THE ORIGIN OF THE ADULT INTESTINAL STEM CELLS. To determine the origin of the adult intestinal stem cells, we made use of transgenic Xenopus tadpoles expressing GFP for recombinant organ cultures. The larval epithelium separated from the wild-type (Wt) or GFP transgenic (Tg) intestines before metamorphic climax was recombined with homologous and heterologous non-epithelial tissues and was cultivated in the presence of TH. In all kinds of recombinant intestines, adult progenitor cells expressing markers for intestinal stem cells such as sonic hedgehog became detectable and then differentiated into the adult epithelium expressing intestinal fatty acid binding-protein, a marker for absorptive cells. Importantly, whenever the epithelium was derived from Tg intestine, both the adult progenitor/stem cells and their differentiated cells expressed GFP, while neither of them expressed GFP in the Wt-derived epithelium. Our results thus have provided direct evidence that stem cells that generate the adult intestinal epithelium originate from the larval epithelium, presumably through TH-induced dedifferentiation of larval epithelial cells.